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Why can radon in groundwater be harmful after it is released into air?

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Diagram of a water treatment plant showing the process from water inlet through mixing, coagulation, flocculation, settling, filtering, chemical treatment with chlorine, storage, and high-lift pumping to the distribution system. — water purification Water from inlets located in the water supply, such as a lake, is sent to be mixed, coagulated, and flocculated and is then sent to the waterworks for purification by filtering and chemical treatment. After being treated, the water is pumped into water mains for storage or distribution.

Radon in groundwater becomes a public health concern when it is released into the air, primarily because it poses a significant risk for lung cancer through inhalation.12

When radon gas—which is soluble in water—is released from well water into the indoor environment, it can accumulate in poorly ventilated spaces. Once in the air, the radon undergoes radioactive decay, producing short-lived radioactive "daughters" such as polonium, bismuth, and lead isotopes. These decay products can attach to dust particles in the air, which are then breathed into the lungs.1

The inhalation of these radioactive particles exposes lung tissue to ionizing radiation, which can damage cells and increase the risk of developing cancer over many years of exposure. Because of this, radon is considered a leading cause of lung cancer among nonsmokers.12